Transistors, such as field effect transistors (FETs), having high breakdown voltages (e.g., above about 15 to about 80 volts or greater) are used in various applications, such as power management or amplification and for driver systems. For example, the breakdown voltage may be defined as the voltage at which the drain (or source) breaks down while the transistor is turned off. In addition, transistors having high breakdown voltages may be used on the periphery of a memory device. For example, these transistors can be located between charge pumps and the string drivers of a memory device that provide voltages to the access lines (e.g., word lines) and can be used in charge pump circuitry and for the string drivers.
One technique for creating transistors with high breakdown voltages uses a lightly doped region between the drain and control gate of the transistor. This region is sometimes referred to as a drain extension region. Devices that use this technique are sometimes referred to as Reduced Surface Field (RESURF) devices.
Transistors with high breakdown voltages sometimes have different breakdown-voltage versus drain-extension-region-doping-level curves (e.g., doping curves), as shown in FIG. 1. For example, doping curve 100 may be for one transistor and the doping curve 100′ may be for another transistor.
It is sometimes desirable to dope the transistors concurrently in a single doping step during fabrication in that multiple doping steps add process steps and thus fabrication costs. The problem with this is that the doping may correspond to the peak breakdown voltage for one transistor (e.g., as indicated by point A), whereas that doping may correspond a relatively low breakdown voltage for the other transistor (e.g., as indicated by point A′). For example, the breakdown voltage at point A′ may be too low for the intended application. Therefore, the doping is sometimes adjusted to compromise so that all of the transistors have breakdown voltages that are sufficient for their intended applications.
For example, the doping may be adjusted to correspond to the point B, where doping curves 100 and 100′ cross and where the breakdown voltage is sufficient for the applications intended for the respective transistors. Note that point B corresponds to over doping (e.g., the doping exceeds that which produces the maximum breakdown voltage) the transistor with doping curve 100 and under doping (e.g., the doping is below that which produces the maximum breakdown voltage) the transistor with doping curve 100′.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative transistor structures and methods of their formation.